Program Brief

Statement of Problem: The client needed a saturated zone flow model of the Yucca Mountain region that could be used to evaluate the effects on flow paths of various alternative conceptual models of hydrogeologic structure and hydrologic boundary conditions.

Approach and Accomplishments: UA three-dimensional groundwater flow model was constructed for the area surrounding Yucca Mountain using the MODFLOW code. The computational grid for this model covers a 28.5 × 41.4-km area that extends vertically from 1,200 m above sea level to 1,500 m below sea level. The model contains 30 horizontal grid layers, varying in thickness from 50 m to 200 m, with the thinnest grid layers assigned at and below the water table where flow paths from Yucca Mountain might occur. Each of the 30 layers is uniformly divided into 300-m horizontal squares for a total of 393,300 computational cells. An algorithm was developed to assign hydrologic properties to each model grid cell based on correspondence to hydrogeologic units and fault zones identified in an Earth Vision geographic information system database for the region. Several calibrated versions of the model, representing alternative sets of boundary conditions, analytical solutions (e.g., confined vs. unconfined), recharge rates, and fault zone properties, were developed to evaluate the effects of model uncertainties on projected flow paths from the proposed nuclear waste repository at Yucca Mountain. Flow paths were evaluated using the MODPATH particle-tracking algorithm, a companion code to MODFLOW.

Client Benefits: The results of these modeling analyses are used to develop simplified abstractions of groundwater flow paths and velocities that are incorporated into risk-based total-system performance assessments to evaluate relative importance of various model and parameter uncertainties. Such analyses permit focusing of research activities on those uncertainties that are shown to be most important. Other uses of the model include delineation of well capture zones and studying the potential effects of future climate changes on groundwater flow paths and velocities.